An MRI apparatus for taking a medical diagnosis image from a nuclear magnetic resonance (NMR) signal can obtain not only morphological images, but also images of the functions and metabolism of living bodies, and has an excellent function of detecting the seat of a disease.
An MRI apparatus using a superconducting magnet having an open structure disclosed in Patent Document 1 has a feature that the magnetic field intensity is uniform and strong, and also the space in the magnet in which an object is disposed brings a bright and open impression.
However as compared with a cylindrical superconducting magnet, in the superconducting magnet having the open structure, the number of support mechanisms for superconducting coils mounted in a cryostat is large and the surface area of the cryostat is also large, so that heat frequently immerses into the cryostat.
Therefore, Patent Document 2 discloses that a heat shield plate for shielding radiation heat from the outside is installed in a multiple style in the cryostat, and also a cryocooler having a cooling capacity for re-cooling vaporized helium gas to its boiling point or less so that the vaporized helium gas is returned to liquid helium is secured to the cryostat.
The cooling capacity of the cryocooler is lowered due to abrasion of movable portions, accumulation of impurities, etc. with lapse of the operation time. However, it is a required condition that it keeps the cooling capacity for reducing helium to the boiling point (4.2 Kelvin temperature) or less, and thus it is necessary to perform a maintenance work to keep and recover the cooling capacity.
Ordinarily, the maintenance work of the cryocooler is carried out every time an operation time of about 10,000 hours elapses, and a filtering unit of a helium compressor connected to the cryocooler is exchanged every about 20,000 hours. An operator checks the time displayed on an integrating operation time indicator provided to the helium compressor, thereby managing the operation time of the cryocooler.
The integrated operation time indicator of the helium compressor is effective to grasp an accurate operation time excluding a time for which the apparatus is stopped due to power outage or the like, and thus there is an advantage that it is possible to synchronously perform the maintenance work on the cryocooler every about 10,000 hours and on the helium compressor every about 20,000 hours by performing the management on the basis of the above time.
Furthermore, Patent Document 3 discloses a superconducting magnet in which a pressure sensor and a heater are disposed in a helium tank having a cryostat mounted therein. In this construction, when the pressure sensor detects that the pressure in the helium tank is set to negative pressure with respect to the ambient pressure, the heater is heated in the helium tank to evaporate liquid helium, whereby the pressure in the helium tank is controlled not to be set to the negative pressure.    Patent Document 1: JP-A-2002-336216    Patent Document 2: JP-A-2005-237417    Patent Document 3: JP-A-6-283329
However, in the performance of the cryocooler, in actual practice some degree of dispersion exists within the duration of the cooling capacity among respective units, and in the case of a cryocooler whose cooling capacity reduces within 10,000 hours and is required to be subjected to maintenance, the execution of the maintenance is delayed.
The function of the cryocooler is to cool helium gas to its boiling point or less so that it is returned to liquid helium. Accordingly, even when degradation of the cooling capacity varies continuously and gently, it cannot liquefy helium at the point in time when the cooling capacity is degraded from the boiling point of helium of 4.2 Kelvin temperature slightly by 0.1°, and thus the cryostat consumes a large amount of helium suddenly.
In the subject matter of Patent Document 3 described above, the pressure in the cryostat can be controlled not to be set to the negative pressure by heating the heater in the cryostat. However, when degradation of the cooling capacity of the cryocooler progresses faster than expected, it would be impossible to detect the degradation concerned, and thus the maintenance timing cannot be judged.
Therefore, it is considered that the maintenance timing of the cryocooler is set to a time shorter than 10,000 hours, without variation. However, as described above, the cooling capacity of the cryocooler has some dispersion every unit, and the maintenance work must be executed on even a cryocooler which keeps sufficient cooling capacity. Therefore, this is undesirable.